Shape analysis of moss (Bryophyta) sporophytes: Insights into land plant evolution

The alternation of generations life cycle represents a key feature of land-plant evolution and has resulted in a diverse array of sporophyte forms and modifications in all groups of land plants. We test the hypothesis that evolution of sporangium (capsule) shape of the mosses-the second most diverse land-plant lineage-has been driven by differing physiological demands of life in diverse habitats. This study provides an important conceptual framework for analyzing the evolution of a single, homologous character in a continuous framework across a deep expanse of time, across all branches of the tree of life.

We reconstruct ancestral sporangium shape and ancestral habitat on the largest phylogeny of mosses to date, and use phylogenetic generalized least squares regression to test the association between habitat and sporangium shape. In addition, we examine the association between shifts in sporangium shape and species diversification.

We demonstrate that sporangium shape is convergent, under natural selection, and associated with habitat type, and that many shifts in speciation rate are associated with shifts in sporangium shape.

Our results suggest that natural selection in different microhabitats results in the diversity of sporangium shape found in mosses, and that many increasing shifts in speciation rate result in changes in sporangium shape across their 480 million year history. Our framework provides a way to examine if diversification shifts in other land plants are also associated with massive changes in sporophyte form, among other morphological traits.